This invention relates generally to nuclear reactors and more particularly, to a handling assembly for the removal and installation of a control rod drive (CRD) in a nuclear reactor.
A reactor pressure vessel (RPV) of a boiling water reactor (BWR) includes a generally cylindrical shape and is closed at both ends, e.g., by a bottom head and a removable top head. A reactor core is supported within the RPV. A plurality of control rods are positioned in the reactor core and operated from below. Control rods control the excess reactivity in the reactor. Specifically, the reactor core contains control rods that assure safe shutdown and provide the primary mechanism for controlling the reactor. The total number of control rods varies with core size and geometry, and is typically between 50 and 200. The positioning of the control rods, for example, fully inserted, fully withdrawn, or somewhere between, is performed by a plurality of control rod drives (CRDs).
A plurality of openings is formed in the bottom head so that components, such as the CRDs, can extend into the RPV. A control rod drive housing extends through the bottom head opening to facilitate securing the CRD through the RPV. The CRD is coupled to the control rod and facilitates positioning the control rod within the core. The CRD extends from the control rod drive housing into an under vessel area.
On occasions, CRDs are removed for maintenance, repair, or replacement. Each CRD is carefully controlled as it is removed from the control rod drive housing and rotated from a generally vertical position to a generally horizontal position for transport out of the under vessel area. These activities should be performed by a minimum number of personnel and accomplished quickly and efficiently to reduce radiation exposure.
At least some known systems for handling a CRD includes a rigid tower system that allows the CRD to be lowered into a tower supported by a trunnion cart. Once the CRD is removed from the CRD housing, the rigid tower system employs a second winch or hoist which rotates the CRD and tower as a unit from vertical to horizontal. The trunnion cart allows the tower, with the secured CRD, to be rotated. In most cases, the tower with the CRD is then removed from the under vessel area. The CRD is raised and lowered by an elevator secured within the tower. At least some known tower systems include complex systems within the tower to raise and lower the elevator.
In one aspect, a positioning apparatus is provided for a CRD handling assembly for a nuclear reactor. The positioning apparatus includes at least one linear slide rail, a drive screw coupled to the at least one slide rail, an elevator movably coupled to the drive screw; and at least one linear bearing fixed to the elevator and slidably coupled to the at least one slide rail.
In another aspect, a handling assembly is provided for a CRD for a nuclear reactor. The handling assembly includes a trunnion cart, a tower coupled to the trunnion cart, a drive screw coupled to the tower, an elevator movably coupled to the drive screw, and at least one slide rail coupled to the tower, the at least one slide rail aligning the elevator.